Solid state lighting devices with selected thermal expansion and/or surface characteristics, and associated methods

ABSTRACT

Solid state lighting devices with selected thermal expansion and/or surface characteristics, and associated methods are disclosed. A method in accordance with a particular embodiment includes forming an SSL (solid state lighting) formation structure having a formation structure coefficient of thermal expansion (CTE), selecting a first material of an interlayer structure to have a first material CTE greater than the substrate CTE, and selecting a second material of the interlayer structure based at least in part on the second material having a second material CTE less than the first material CTE. The method can further include forming the interlayer structure over the SSL formation structure by disposing (at least) a first layer of the first material over the SSL formation structure, a portion of the second material over the first material, and a second layer of the first material over the second material. The SSL formation structure supports an SSL emitter material, and the method further includes counteracting a force placed on the formation structure by the first material, by virtue of the difference between the second material CTE and the first material CTE. In other embodiments, the SSL formation structure can have an off-cut angle with a non-zero value of up to about 4.5 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.12/861,706 filed on Aug. 23, 2010, and claims priority to U.S.Provisional Application No. 61/236,300 filed on Aug. 24, 2009, and U.S.Provisional Application No. 61/261,065 filed on Nov. 13, 2009, each ofwhich is incorporated herein by reference. To the extent that anymaterial in the present application conflicts with the disclosures ofthe foregoing Provisional Applications, the material of the presentapplication controls.

TECHNICAL FIELD

The present disclosure is related to solid state lighting (“SSL”)devices with selected thermal expansion characteristics, and/or surfacecharacteristics, and associated methods, including methods ofmanufacturing.

BACKGROUND

Mobile phones, personal digital assistants (“PDAs”), digital cameras,MP3 players, and other portable electronic devices utilize SSL devices(e.g., LEDs) for background illumination. SSL devices are also used forsignage, indoor lighting, outdoor lighting, and other types of generalillumination. FIG. 1A is a cross-sectional view of a conventional SSLdevice 10 a with lateral contacts. As shown in FIG. 1A, the SSL device10 a includes a substrate 20 carrying an LED structure 11 having anactive region 14, e.g., containing gallium nitride/indium galliumnitride (GaN/InGaN) multiple quantum wells (“MQWs”), positioned betweenN-type GaN 15, and P-type GaN 16. The SSL device 10 a also includes afirst contact 17 on the P-type GaN 16 and a second contact 19 on theN-type GaN 15. The first contact 17 typically includes a transparent andconductive material (e.g., indium tin oxide (“ITO”)) to allow light toescape from the LED structure 11. FIG. 1B is a cross-sectional view ofanother conventional LED device 10 b in which the first and secondcontacts 17 and 19 are opposite of each other, e.g., in a verticalrather than lateral configuration. In the LED device 10 b, the firstcontact 17 typically includes a reflective and conductive material(e.g., aluminum) to direct light toward the N-type GaN 15.

As discussed in more detail below, the various elements of the SSLdevices typically have different coefficients of thermal expansion(CTE). During temperature excursions that occur in manufacturingprocesses and/or during use, the difference in CTEs of the deviceelements may cause the elements to delaminate. In addition, as is alsodiscussed in more detail below, several elements of the SSL device aregrown epitaxially on the substrate 20. It is accordingly desirable tocontrol the growth of the materials forming these elements in a mannerthat improves the performance and reliability of the resulting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional diagram of an SSL device inaccordance with the prior art.

FIG. 1B is a schematic cross-sectional diagram of another SSL device inaccordance with the prior art.

FIG. 2A is a cross-sectional view of an SSL device in accordance withembodiments of the present technology.

FIG. 2B is a schematic illustration of a portion of an SSL substratethat is off-cut in accordance with an embodiment of the presenttechnology.

FIG. 2C is a graph illustrating a range of off-cut angles suitable foruse with SSL substrates in accordance with the present technology.

FIG. 3A is an enlarged illustration of waves formed in an overlayer onan SSL substrate having a 4.5° off-cut angle.

FIG. 3B is an enlarged illustration of an overlayer on an SSL substratehaving a 0.5° off-cut angle.

FIG. 4A is a further enlarged view of the structure shown in FIG. 3A.

FIG. 4B is a further enlarged view of the structure shown in FIG. 3B.

FIG. 5A is a schematic illustration of a process for bonding an SSLsubstrate to a support member in accordance with the present technology.

FIG. 5B is a schematic illustration of the structure resulting from theprocess conducted in FIG. 5A.

FIGS. 5C-5E illustrate a process for forming an interlayer structure inaccordance with the present technology.

FIG. 6A is a schematic illustration of forces on elements of theinterlayer structure shown in FIG. 5E.

FIG. 6B is a schematic illustration of an interlayer structure havingelements arranged in accordance with another embodiment of the presenttechnology.

FIG. 7 is a schematic illustration of elements of an SSL deviceconfigured in accordance with an embodiment of the present technology.

DETAILED DESCRIPTION

Various embodiments of SSL devices with particularly selected thermalexpansion coefficients and/or surface characteristics, and associatedmethods are described below. As used hereinafter, the term “SSL device”generally refers to devices with light emitting diodes (“LEDs”), organiclight emitting diodes (“OLEDs”), laser diodes (“LDs”), polymer lightemitting diodes (“PLEDs”), and/or other suitable sources of illuminationother than electrical filaments, a plasma, or a gas. A person skilled inthe relevant art will also understand that the technology may haveadditional embodiments, and that the technology may be practiced withoutseveral of the details of the embodiments described below with referenceto FIGS. 2A-7.

FIG. 2A is a schematic cross-sectional diagram of an SSL device 110 withlateral contacts in accordance with embodiments of the technology. Asshown in FIG. 2A, the SSL device 110 can include an SSL formationstructure 120 carried by a support member 130. The SSL device 110 canfurther include an optional buffer material 152. The SSL formationstructure 120 carries an SSL structure 111 that includes an activeregion 114 (e.g., an SSL emitter material), positioned in series betweena first semiconductor material 115 and a second semiconductor material116. The SSL device 110 can also include a first contact 117 on thefirst semiconductor material 115 and a second contact 119 on the secondsemiconductor material 116 to provide power to the SSL structure 111. Inthe illustrated embodiment, the first and second contacts 117, 119 arearranged laterally relative to each other. In other embodiments, thecontacts 117, 119 can be arranged vertically relative to each other, orcan have other suitable configurations. In any of these embodiments, theSSL device 110 can optionally include a reflective material (e.g., asilver film), a carrier material (e.g., a ceramic substrate), an opticalcomponent (e.g., a collimator), and/or other suitable components forenhancing the efficiency and/or other characteristics of the SSL device110, including but not limited to the quality of the emitted light.

In certain embodiments, the SSL formation structure 120 can includesilicon (Si), at least a portion of which has the Si(1,1,1) crystalorientation. In other embodiments, the formation structure 120 caninclude silicon with other crystal orientations (e.g., Si(1,0,0)),aluminum gallium nitride (AlGaN), GaN, silicon carbide (SiC), sapphire(Al₂O₃), zinc oxide (ZnO₂), a combination of the foregoing materialsand/or other suitable materials. In the illustrated embodiment, the SSLformation structure 120 has a first surface 121 proximate to theoptional buffer material 152 that is off-cut, as described in furtherdetail later with reference to FIGS. 2B-2C. In other embodiments, thefirst surface 121 of the formation structure 120 may have othercharacteristics, e.g., openings, channels, and/or other surfacefeatures, not shown in FIG. 2A.

The optional buffer material 152 can facilitate the formation of thefirst and second semiconductor materials 115, 116 and the active region114 on the SSL formation structure 120. In certain embodiments, theoptional buffer material 152 can include at least one of aluminumnitride (AlN), AlGaN, zinc nitride (ZnN), GaN, and/or other suitablematerials. In other embodiments, the optional buffer material 152 may beomitted, and the first semiconductor material 115 may be formed directlyon the formation structure 120, or on an intermediate interlayerstructure, which is described in more detail later with reference toFIGS. 5D-6B.

The first and second semiconductor materials 115, 116 can be configuredas cladding components for the active region 114. In certainembodiments, the first semiconductor material 115 can include N-type GaN(e.g., doped with silicon (Si)), and the second semiconductor material116 can include P-type GaN (e.g., doped with magnesium (Mg)). In otherembodiments, the first semiconductor material 115 can include P-typeGaN, and the second semiconductor material 116 can include N-type GaN.In further embodiments, the first and second semiconductor materials115, 116 can each include at least one of gallium arsenide (GaAs),aluminum gallium arsenide (AlGaAs), gallium arsenide phosphide (GaAsP),gallium(III) phosphide (GaP), zinc selenide (ZnSe), boron nitride (BN),AlGaN, and/or other suitable semiconductor materials. Additionally, theP-type GaN and/or N-type GaN can also be doped with silicon.

The active region 114 can include a single quantum well (“SQW”),multiple quantum wells (“MQWs”), and/or a bulk semiconductor material.As used hereinafter, a “bulk semiconductor material” generally refers toa single grain semiconductor material (e.g., InGaN) with a thicknessgreater than about 10 nanometers and up to about 500 nanometers. Incertain embodiments, the active region 114 can include an InGaN SQW,GaN/InGaN MQWs, and/or an InGaN bulk material. In other embodiments, theactive region 114 can include aluminum gallium indium phosphide(AlGaInP), aluminum gallium indium nitride (AlGaInN), and/or othersuitable materials or configurations.

In certain embodiments, the first semiconductor material 115, the activeregion 114, the second semiconductor material 116, and the optionalbuffer material 152 can be formed on the formation structure 120 viametal organic chemical vapor deposition (“MOCVD”), molecular beamepitaxy (“MBE”), liquid phase epitaxy (“LPE”), and/or hydride vaporphase epitaxy (“HVPE”). In other embodiments, at least one of theforegoing components may be formed via other suitable epitaxial growthtechniques. As explained in more detail below, significant internalstresses are induced between the formation structure 120 and at leastthe first semiconductor material 115 as the assembly cools followingepitaxial processes.

In certain embodiments, the first contact 117 can include copper (Cu),aluminum (Al), silver (Ag), gold (Au), platinum (Pt), and/or othersuitable conductive material. In other embodiments, the first contact117 can include ITO, aluminum zinc oxide (“AZO”), fluorine-doped tinoxide (“FTO”), and/or other suitable transparent and conductive oxide(“TCOs”). Techniques for forming the first contact 117 can includeMOCVD, MBE, spray pyrolysis, pulsed laser deposition, sputtering,electroplating, and/or other suitable deposition techniques. The secondcontact 119 can include a suitable conductive material 112 and asuitable contact material 113 between the second semiconductor material108 and the conductive material 112. The conductive material 112, forexample, can be a transparent conductive material.

FIG. 2B is a partially schematic, enlarged illustration of a portion ofan SSL substrate 126 that is off-cut in accordance with embodiments ofthe present technology. The substrate 126 is exfoliated in a subsequentprocess step to produce the SSL formation structure 120 described above.The SSL substrate 126 has a first surface 121 and a second surface 122facing away from the first surface 121. The first surface 121 (and, inat least some embodiments, the second surface 122) is off-cut usingtechniques known to those of ordinary skill in the relevant art, at aselected off-cut angle A. Off-cutting the SSL substrate 126 in thismanner can produce a series of terraces 123 at the first surface 121,each having a terrace length L.

FIG. 2C is a graph illustrating the terrace length L as a function ofthe off-cut angle A. FIG. 2C illustrates that for shallow off-cut anglesA, the terrace length L is relatively large, and for steeper off-cutangles A, the terrace length L is relatively short. In particularembodiments, off-cut angles A between A1 and A2 produce beneficialeffects describes further below. In a further particular aspect of theseembodiments, the lower bound of the off-cut angle range A1 is anynon-zero value, and in yet a further particular aspect, has a value ofabout 0.5°. The upper bound of the off-cut angle range, identified byangle A2, can have a value of less or at least not more than about 4.5°,and in a particular embodiment, an angle of about 4°. In still furtherparticular embodiments, A2 can have a value of about 2°.

FIGS. 3A and 3B compare the results obtained when materials are disposedon SSL substrates having different off-cut angles. For example, FIG. 3Aillustrates a first SSL substrate having an overlayer 150 disposed onthe first surface. In a particular embodiment, the first SSL substratecan include a 4 inch silicon wafer, and the overlayer 150 can includetwo-micron n-GaN with a single silicon nitride (e.g., S₃N₄ or SiNx)interlayer. The first SSL substrate has been off-cut at an angle of4.5°, and produces a series of waves 151 in the overlayer 150, whichhave been enhanced for clarity in FIG. 3A. By contrast, a second SSLsubstrate having a 0.5° off-cut angle (shown in FIG. 3B) produces novisible waves at the same level of magnification.

FIGS. 4A and 4B are further enlarged illustrations of portions of thefirst SSL substrate and the second SSL substrate, respectively. FIG. 4Aillustrates a representative one of the waves 151 produced in theoverlayer 150 when the first SSL substrate is off-cut at an angle of4.5°. FIG. 4B illustrates an analogous region of the overlayer 150disposed on the second SSL substrate (off-cut at 0.5°), which has muchsmaller or even no waves at the same level of magnification.Accordingly, a method in accordance with the present technology includesselecting an off-cut angle that is less than an angle expected toproduce a threshold level of waves in the overlayer 150. In oneembodiment, the threshold level is zero and in other embodiments, thethreshold level has a non-zero value.

It is expected that by off-cutting the SSL substrate 126 at a valuewithin the foregoing ranges (e.g., a non-zero value less than 4.5°, or avalue of between about 0.5° and about 2°) the characteristics of layersdisposed on the resulting SSL formation structure 120 can be enhanced.In particular, it is expected that the crystal structures grown on theSSL formation structure 120, such as AN and/or GaN, can be aligned in amore stable state. As a result, it is expected that the aligned crystalstructures produced by the appropriate degree of off-cut may reducedefects in subsequently formed layers. For example, the foregoingmethodology can improve the uniformity and alignment of one or more GaNlayers applied to the SSL formation structure 120 during the formationof LED devices. In selected embodiments, the higher temperature and/orincreased nitrogen-to-gallium ratio present during the formation of SiNxin an interlayer structure may intensify the step bunching effects thatcreate the waves shown in FIGS. 3A and 4A. Selecting a non-zero off-cutangle not greater than 4.5°, and generally less than 4.5°, can reduce oreliminate this effect. The amount of SiNx in the interlayer structurecan determine the off-cut angle. For example, the SiNx can bedistributed relatively thickly to cover more surface area for SSLformation structures with lower off-cut angles, and more thinly to coverless surface area for SSL formation structures with higher off-cutangles. Further steps in the interlayer formation process are describedbelow with reference to FIG. 5A-5E.

Referring now to FIG. 5A, an SSL substrate 126 having an off-cut firstsurface 121′ is shown positioned over a support member 130. The supportnumber 130 can include a suitable material having a coefficient ofthermal expansion (CTE) selected to match or at least suitablyapproximate the CTEs of layers formed on the SSL substrate 126 duringsubsequent processing steps. The SSL substrate 126 may have a CTE thatdiffers significantly from that of the subsequently formed layers, e.g.,when the SSL substrate 126 includes silicon, and the subsequent layerincludes GaN. Accordingly, the CTE of the support member 130 can beselected to be closer to the CTE of the subsequent layers than is theCTE of the SSL substrate 126, and the thickness of the support member130 can be large enough relative to the thickness of the formationstructure obtained from the SSL substrate 126 to control the thermalexpansion of the thin SSL formation structure 126. Accordingly, when thesubsequently-formed layers include GaN, the support member 130 caninclude GaN, molybdenum, or, in particular embodiments, polyaluminumnitride (pAlN), which is expected to control the thermal expansion ofthe SSL formation structure to better approximate that of the buffermaterial and/or the subsequently-formed semiconductor materials in acost-effective manner.

A first bonding layer 124 a is disposed on the second surface 122 of theSSL substrate 126, and a corresponding second bonding layer 124 b isdisposed on the support member 130. As shown in FIG. 5B, the two bondinglayers 124 a, 124 b are brought into contact with each other to form abond region 131 between the SSL substrate 126 and the support member130. Depending on the application, a portion of the substrate 126 (shownin dotted lines) is removed using known exfoliation processes to leave athin SSL formation structure 120 of the substrate 126 bonded to thesupport substrate 130. The SSL formation structure 120 has an exposedfirst surface 121.

FIG. 5C illustrates the formation of an optional buffer material 152 onthe first surface 121 of the SSL formation structure 120. The buffermaterial 152 can include AN, AlGaN, or another suitable material that isgrown on the SSL formation structure 120. As discussed above, theoff-cut angle at the first surface 121 can align the crystals of thebuffer material 152 in a more stable state.

FIGS. 5D and 5E illustrate the formation of an interlayer structure 140carried by the SSL formation structure 120. In a particular embodiment,the interlayer structure 140 is formed on the buffer material 152. Inother embodiments for which the buffer material 152 is omitted, theinterlayer structure 140 can be formed directly on the SSL formationstructure 120. The interlayer structure 140 includes at least twomaterials, shown in FIG. 5D as a first material 141 and a secondmaterial 142. In a representative embodiment, the first material 141includes GaN, and the second material 142 includes silicon nitride,though in other embodiments these materials can include otherelements/compounds. The first material 141 is grown over the SSLformation structure 120 to form a generally uniform layer. The secondmaterial 142 typically does not form a uniform, continuous layer, butinstead forms discontinuous, discrete and/or spaced-apart volumes on thefirst material 141. One purpose of the second material 142 is to blockdislocations or other defects formed in the underlying layer of thefirst material 141 from propagating into subsequently grown layers ofthe first material 141 or other materials grown on the underlying layersof the first material 141. Accordingly, multiple layers of the firstmaterial 141 may be grown in a stacked manner on the SSL formationstructure 120, each with a successively greater layer thickness andreduced defect level, and each separated from its neighbor by a quantityof the second material 142, until a first material layer with anadequate thickness and an acceptably low number and/or density ofdefects is obtained.

In FIG. 5E, a second layer of the first material 141 is disposed on thesecond material 142 and (where the second material 142 does not coverthe underlying first material 141), on the underlying first material141. The combination of the two first material layers 141 and theinterposed second material 142 forms a representative interlayerstructure 140. The foregoing process may be repeated to produceadditional layers of the first material 141, with each successive firstmaterial layer generally having fewer defects (by virtue of the blockingeffect provided by the second material 142 below) and a successivelyincreasing thickness.

FIG. 6A is an enlarged illustration of a portion of the interlayerstructure 140 shown in FIG. 5E, with characteristics of the first andsecond materials 141, 142 selected to enhance the resulting SSL device.In particular embodiment, the first material 141 (e.g., GaN) has a CTEthat is significantly different (e.g., higher) than the CTE of the SSLformation structure 120. Accordingly, when the resulting SSL device 110(FIG. 2A) undergoes temperature excursions, the layers within the SSLdevice 110 will expand or contract by different amounts and inducedelamination forces between the different materials. The buffermaterial, semiconductor materials, and interlayer structure 140 areformed at high temperatures. As a result, when the resulting SSL device110 is cooled after the epitaxial or other high temperature processes,the first material 141 will contract more than the SSL formationstructure 120. This induces tension in the first material 141 andcompression in the SSL formation structure 120. By selecting the CTE ofthe second material 142 to be less than that of the first material 141,the second material 142 can provide a counterforce that reduces thetendency for the layers of the first material 141 to deform, delaminate,and/or otherwise undergo a damaging or destructive process. Accordingly,the force on the first material 141 (indicated by arrows F1) can becounteracted by the oppositely directed force provided by the secondmaterial 142 (indicated by arrows F2) to produce the desired compositeCTE. For example, the second material 142 can be selected to have a CTEthat is less than that of the first material 141, but still greater thanthat of the SSL formation structure 120. In a particular example, thesecond material 142 can include SiNx when the first material 141includes GaN. In other embodiments, these materials can have othercompositions. In still further embodiments, the second material 142 canbe selected to have a CTE that is less than that of both the firstmaterial 141 and the SSL formation structure 120, so long as thecombined or composite CTE of the first material 141 and the secondmaterial 142 is not so far below the CTE of the SSL formation structure120 as to result in compressive (rather than tensile) stresses in thefirst material 141.

The technique for manufacturing the interlayer structure 140 can includeother features that also reduce the stresses in the first material 141.For example, the first material 141 can be doped with silicon and/oranother suitable material, e.g. in addition to interspersing the secondmaterial 142 between layers of the first material 141. In otherembodiments, the first material can be doped with other constituents ofa suitable SSL formation structure 120.

In a particular embodiment described above with reference to FIGS.5D-5E, the first material 141 is positioned directly on the buffermaterial 152, or, if the buffer material 152 is omitted, directly on theSSL formation structure 120. In another embodiment shown in FIG. 6B, thesecond material 142 can be formed on the buffer material 152 or, if thebuffer material 152 is omitted, directly on the SSL formation structure120. In either embodiment, the interlayer structure 140 includes atleast two layers of the first material 141, with the second material 142disposed between the two layers.

In any of the foregoing embodiments, the forces on the first material141 can be reduced more effectively by locating the second material 142close to the SSL formation structure 120. Accordingly, it may bebeneficial to dispose the second material 142 directly on the SSLformation structure 120, or directly on the buffer material 152. If thefirst quantity of the second material 142 in the interlayer structure isnot disposed directly on the SSL formation structure 120 or directly onthe buffer material 152, it may nevertheless be beneficial to disposethe second material 142 in close proximity to both the SSL formationstructure 120 and the buffer material 152, e.g., within 300 nm of thebuffer material 152.

FIG. 7 is a schematic illustration of the SSL device 110 after the SSLstructure 111 is formed on the interlayer structure 140. The SSLstructure 111 can include a first semiconductor material 115 (e.g., anN-type GaN), an active region 114 (e.g., including InGaN), and a secondsemiconductor material 116 (e.g., a P-type GaN). The SSL structure 111can further include a first contact 117 (e.g., a P-type contact). In aparticular embodiment, a portion of the first contact 117, the secondsemiconductor material 116, and active region 114 can be removed toexpose a portion of the first semiconductor material 115 below, thusallowing the formation of the second contact 119, as shown in FIG. 2A.In this embodiment, the entire structure can be packaged andincorporated into an end-user device, or the interlayer structure 140and elements below it can be separated from the rest of the SSL device110 as indicated by a separation line 118. If the SSL device 110 isseparated at the separation line 118, the lower surface of the firstconductive material 115 is exposed, allowing the formation of the secondcontact in a vertical rather than lateral orientation. In any of theforegoing embodiments, it is expected that selecting the first andsecond materials 141, 142 of the interlayer structure 140, and/orselecting the appropriate off-cut angle for the SSL substrate 126 andresulting formation structure 120 can improve the reliability,efficiency, and/or producability of the resulting SSL device 110.

From the foregoing, it will be appreciated that specific embodiments ofthe technology have been described herein for purposes of illustration,but that various modifications may be made without deviating from thetechnology. For example, the interlayer structure can have differentnumbers and/or arrangements of layers than shown in the Figures,depending upon the specific implementation and/or other factors.Materials other than SiNx can be used to provide the counterforce on theGaN (or other) layers of the interlayer structure. Such materialsinclude, but are not limited to, silicon oxide, aluminum oxide andgallium oxide. Certain aspects of the technology described in thecontext of particular embodiments may be combined or eliminated in otherembodiments. For example, the buffer material 152 may be eliminated insome embodiments. In some embodiments, the SSL device 110 can include aninterlayer structure having the first and second materials selected onthe basis of CTE characteristics, without also including an off-cut SSLformation structure. In other embodiments, the SSL device can include anoff-cut SSL formation structure, without an interlayer structure thatincludes materials selected based on CTE characteristics. Further, whileadvantages associated with certain embodiments of the technology havebeen described in the context of those embodiments, other embodimentsmay also exhibit such advantages, and not all embodiments neednecessarily exhibit such advantages to follow within the scope of thepresent disclosure. Accordingly, the disclosure and associatedtechnology can encompass other embodiments not expressly shown ordescribed herein.

I/we claim:
 1. A solid state lighting (SSL) device, comprising: asupport member; an SSL formation structure carried by the supportmember, the SSL formation structure having a first surface facing towardthe support member and a second surface facing oppositely away from thefirst surface, the second surface having an off-cut angle with anon-zero value up to about 4.5°; and an SSL structure carried by the SSLformation structure, the SSL structure including a P-type region, anN-type region, and an active region between the P-type region and theN-type region.
 2. The device of claim 1, wherein the off-cut angle has avalue of from about 0.5° to about 2°.
 3. The device of claim 1, furthercomprising a buffer material positioned between the SSL formationstructure and the SSL structure.
 4. The device of claim 3, furthercomprising an interlayer structure between the buffer material and theSSL formation structure, wherein the interlayer structure includes atleast two layers of gallium nitride and a quantity of silicon nitridebetween the two layers of gallium nitride.